The invention relates to a method for fabricating synthetic-material molded parts with sharp-edged, strong contours or undercuts, preferably supporting or covering parts for motor vehicles containing recesses and/or break-throughs or passages, through blow molding a freshly extruded synthetic-material tube. The invention also relates to a device for implementing the method.
Synthetic-material molded parts, which may also be coated or partly coated, are needed in the automobile industry, e.g. for dashboards, which are formed with deep recesses or break-throughs with undercuts for accepting or leading through fittings and other functional elements, for example. The realization of those kinds of molded parts as hollow bodies fabricated by the blow molding process makes it possible to achieve high stability combined with flexibility. Despite the advantages of that method, difficulties can arise during the molding of strong, sharp-edged contours or undercuts. That is because during blow molding of a freshly extruded viscous thermoplastic synthetic-material tube, which is inserted into a blow-molding die and pressed against or into internal contours of the blow-molding die through blowing in air, its inner surfaces can touch. That occurs especially in the case of narrow or flat-shaped extruded tubes and contours reaching far into the inside of the die. Therefore, defect-free fabrication of the molded part is no longer guaranteed because of the bonds which result. The danger of unwanted bonds is particularly acute when forming undercuts and in regions with a very small separation between the inner surfaces for subsequent formation of cutouts. If a coating, for example of a textile material, must also be attached to all or to parts of the outer surface of the molded part, it is also possible, especially if the molded part has pronounced contours and undercuts, for the coating material to adhere to the surface of the tube prematurely or for the coating material not to lie properly against the sharp-edged internal contour and be distorted or over-stretched.
In order to avoid the danger of mutual contact between the inner surfaces of the tube and an associated bonding thereof, or a premature contact between the outer surface of the extruded synthetic-material tube and the coating material, a procedure is already known for providing the blow-molding die with drillings connected to a vacuum pump in order to fix the extruded synthetic-material tube or the coating material in a specific position. However, in the case of strongly pronounced contours, that measure is still unable to ensure defect-free fabrication of the molded parts.
It is accordingly an object of the invention to provide a method and a device for using extrusion blow-molding dies to fabricate synthetic-material molded parts, which overcome the hereinafore-mentioned disadvantages of the heretofore-known methods and devices of this general type, in which the molded parts have strong, sharp-edged contours or cutouts with recesses and possibly with a surface coating and in which high-quality finished products can be produced with a low rejection rate.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for fabricating synthetic-material molded parts, preferably supporting or covering parts for motor vehicles containing recesses and/or breakthroughs, through blow-molding, which comprises placing a freshly extruded synthetic-material tube in a blow-molding die having flat, slightly or gently contoured surfaces; initially expanding the synthetic-material tube in the blow-molding die with pneumatic internal pressure and molding the synthetic-material tube onto the surfaces of the blow-molding die; and holding the synthetic-material tube onto the surfaces of the blow-molding die, for shaping strong, sharp-edged contours and undercuts with mechanical pressure exerted by the blow-molding die on an outer surface of the internally pneumatically acted upon and expanded synthetic-material tube.
Thus the freshly extruded synthetic-material tube is expanded in the blow-molding die into an internal space free of strongly pronounced projections, pre-molded and held, so that the existing danger of bonding between the internal surfaces, particularly of narrow synthetic-material tubes, is considerably reduced or even completely eliminated. The sharp-edged recesses or undercuts are then pressed-in during a second molding step through the use of a tool shaped according to the contours to be achieved, which is driven against the outer wall of the expanded synthetic-material tube held in the blow-molding die. High-quality molded parts are thus available with low defect rates.
In accordance with another mode of the invention, the mechanical molding of the contour takes place in several partial steps in order to be able to achieve a gradual, differentiated molding and different contour structures.
In accordance with a further mode of the invention, the inner surfaces of the tube for forming undercuts, which originate from break-throughs to be produced in this region, are more or less strongly pressed together. The actual undercut thereby arises after formation of a break-through in the region which is pressed together.
In accordance with an added mode of the invention, a variable wall thickness in the undercut region is set through selection of the mold pressure.
In accordance with an additional mode of the invention, the different wall thicknesses are produced in one and the same undercut region, or cavities are created or left between two inner wall surfaces, by pressing together in several molding steps.
In accordance with yet another mode of the invention, the entire molding process proceeds in combination with surface-coating of the molded part in such a way that a coating material laid in the blow-molding die is pressed onto the surface and into the sharp-edged contours of the molded part through the pneumatic internal pressure and the tool is driven against the outer surface of the synthetic-material tube. This enables error-free attachment of the coating web, i.e. without premature adhesion, displacement or distortion.
With the objects of the invention in view, there is also provided a device for fabricating synthetic-material molded parts through blow-molding, comprising a blow-molding die having two die halves and a wall defining an interior for receiving an extruded, preformed, synthetic-material tube to be expanded and held on the wall of the blow-molding die; and a shaping slide encased by the wall of the blow-molding die, the shaping slide movable or slidable into the interior of the blow-molding die for mechanically influencing outer wall surfaces of the expanded, preformed synthetic-material tube held on the wall of the blow-molding die.
In accordance with another feature of the invention, the shaping slides are used for shaping the sharp-edged recesses or undercuts in the pre-molded, expanded synthetic-material tube, and a front surface of the shaping slide, i.e. the surface directed towards the inside of the blow-molding die, finishes flush with the inner surface of the blow-molding die before the molding and during removal from the mold.
In accordance with a further feature of the invention, the shaping slide is formed from a combined block of several slide parts which can be driven forward or retracted either synchronously or independently from each another. As a result the molding of the sharp-edged recesses or undercuts can be performed in several partial steps in order to achieve different shapes, layer thicknesses or cavities and a more careful, gentle molding procedure can be carried out.
In accordance with an added feature of the invention, the slide parts are slide lamellae disposed adjacently parallel to each other.
In accordance with an additional feature of the invention, additional slide elements disposed perpendicularly to the slide lamellae can be movably attached at their narrow longitudinal surfaces whereas leading slide elements can be disposed at outside broad surfaces of the slide lamellae.
In accordance with yet another feature of the invention, the slide lamellae and the slide segments, with or without cutting edges, individually serve the purpose of molding and removing the synthetic-material tube from the blow-molding die.
In accordance with yet a further feature of the invention, the shaping slide or the slide lamellae or the slide segments or the slide elements are individually steplessly driven mechanically, hydraulically or pneumatically.
In accordance with yet an added feature of the invention, the wall of the blow-molding die has vacuum drillings formed therein to be connected to a vacuum pump for additionally securing at least one of the synthetic-material tube and a coating material to be pressed to the synthetic-material tube, in position.
In accordance with yet an additional feature of the invention, the shaping slide is a diaphragm. In accordance with a concomitant feature of the invention, the diaphragm is formed of rubber or a different elastic material.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and a device for fabricating synthetic-material molded parts through blow molding, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.